Phase-change heat reservoir device for transient thermal management

ABSTRACT

A heat transfer system is presented for managing thermal transients, thus providing engineers greater flexibility in designing thermal solutions for applications subject to transient heat-generation. A heat reservoir device for managing a heat input subject to transient conditions includes a heat transfer subsystem having a first end and a second end, where the first end is thermally coupled to the heat input; a heat storage subsystem coupled to the second end of the heat transfer subsystem, where the heat storage subsystem comprises a phase change material responsive to the transient conditions. The excess heat load during transient operation is temporarily absorbed by the latent heat of fusion when the phase change material changes its phase from solid to liquid. Subsequently, the absorbed heat can be released back to the ambient via a heat rejection subsystem. This allows engineers to design smaller heat sinks capable of accommodating given transient conditions. This results in heat sinks which are lower cost and smaller size, or which reduce the requirement to provide higher airflow, thereby also decreasing cost and noise, and increasing reliability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This invention claims priority from U.S. Provisional Patent ApplicationSer. No. 60/261,551, filed Jan. 26, 2001, which is hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates, generally, to heat transfer systems and,more particularly, to a method of managing thermal transients throughthe use of phase-change heat reservoirs.

2. Background Information

Recent advances in the design and fabrication of electronic componentshas dramatically increased their speed and density but has, at the sametime, led to significant challenges for thermal engineers seeking toprovide heat-transfer solutions for such components. These heat transferchallenges are particularly severe in applications subject to transientheat generation.

Currently known methods of addressing transient heat generation areundesirable in a number of respects. For example, when confronted bythermal requirements of a system, thermal engineers will typicallyselect the maximum heat load in the transient heat profile as theguideline for designing the thermal solution. This “worst-case-scenario”practice invariably results in an unnecessarily large heat sink orcooler that is not used to its full capacity during normal operation.

Furthermore, if there is a space constraint for the overall system whichdoes not allow the full required space for a traditional thermalsolution, then the engineer must increase the cooling airflow (throughthe use of fans or other forced convection devices) and/or use othermeans to improve performance. This can result in increased noise andother undesirable effects.

While the prior art makes limited use of phase change materials to storetransient thermal energy, such solutions effectively utilize largeheat-sinks having an integral phase-change chamber which is simplyattached to the heat source. This solution fails to address, among otherthings, the space constraints presented by modern systems.

Methods are therefore needed in order to overcome these and otherlimitations of the prior art.

BRIEF SUMMARY OF THE INVENTION

Systems and methods in accordance with the present invention overcomethe prior art by providing a heat transfer system capable of managingthermal transients, thus giving engineers greater flexibility indesigning thermal solutions for applications subject to transientheat-generation. In accordance with one embodiment of the presentinvention, a heat reservoir device for managing a heat input subject totransient conditions comprises: a heat transfer subsystem configured todistribute heat from a heat input to a heat storage subsystem, said heatstorage subsystem comprising a phase change material responsive to saidtransient conditions.

In accordance with one aspect of the present invention, the excess heatload during transient operation is temporarily absorbed by the latentheat of fusion when the phase change material changes its phase fromsolid to liquid. Subsequently, the absorbed heat can be released back tothe ambient via a heat rejection subsystem.

In accordance with another aspect of the present invention, methods areprovided for designing smaller heat sinks capable of accommodating giventransient conditions. This results in heat sinks which are lower costand smaller size, or which reduce the requirement to provide higherairflow, thereby also decreasing cost and noise, and increasingreliability.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject invention will hereinafter be described in conjunction withthe appended drawing figures, wherein like numerals denote likeelements, and:

FIG. 1 is a schematic overview of a heat transfer system in accordancewith one aspect of the present invention;

FIG. 2 is a partial cut-away view of a heat reservoir in accordance withone embodiment of the present invention;

FIG. 3 is a partial cut-away view of a heat reservoir interfacing with aheat collector and heat sink in accordance with one embodiment of thepresent invention;

FIG. 4 is a partial cut-away view of one embodiment of the presentinvention wherein the heat sink and heat reservoir remotely located fromthe heat collector;

FIG. 5 is a partial cut-away view of an alternate embodiment of thepresent invention wherein a heat-sink is integrated with the heatreservoir;

FIG. 6 is a partial cut-away view of an alternate embodiment of thepresent invention wherein the heat storage subsystem includes axialfins;

FIG. 7 is a partial cut-away view of an alternate embodiment of thepresent invention;

FIG. 8 is a partial cut-away view of a heat reservoir wherein axial finsare separated from the heat transfer medium; and

FIG. 9 is a partial cut-away view of a heat reservoir comprising radialpin fins.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Systems and methods in accordance with the present invention overcomethe prior art by providing a heat transfer system capable of managingthermal transients through the use of a phase change heat reservoirdevice.

Overview

Referring now to FIG. 1, a system 100 in accordance with the presentinvention generally includes a heat transfer subsystem 102, a heatrejection subsystem 104, and a heat storage subsystem 106, whereinsystem 100 accepts a heat input 110 subject to transient conditions.Heat storage subsystem 106 is configured to absorb excess thermal loadgenerated during these transient conditions, and heat rejectionsubsystem 104 is configured to transfer the heat away from system 100(i.e., to the ambient environment). Heat transfer subsystem 102 thenprovides the thermal path between the various subsystems 106 and 104 andheat input 110. As will be discussed in detail below, these threesubsystems may comprise a variety of components, and may be arranged ina variety of topologies. Furthermore, certain subsystems (e.g., heatrejection subsystem 104), may not be required in particularapplications. As described in detail below, the combination of heattransfer subsystem 102 and heat storage subsystem 106 is oftencollectively referred to herein as a “heat reservoir”.

Heat transfer subsystem 102 provides one or more thermal paths frominput 110 to and between subsystems 102, 104, and 106. Toward this end,heat transfer subsystem may comprise any suitable heat transfercomponent or components. For example, heat transfer subsystem 102 mayinclude a conductive material, heat-pipe, thermosyphon, liquid coolingloop, or the like, connected to a heat collector which itself contactsor is otherwise thermally coupled to the heat source. Variousembodiments such as these will be discussed below in connection withFIGS. 2–9.

Heat rejection subsystem 104 provides the primary heat path to theexternal environment, and may operate through one or more heat transfermodes, i.e., convection (forced or free), radiation, and/or conduction.For example, heat rejection subsystem 104 may include one or more heatsinks, radiators, coldplates, or the like. Heat storage subsystem 106acts as a thermal storage unit, or “heat capacitor,” and, in conjunctionsubsystems 102 and 104, provides relief from thermal transientspresented by input 110. In a preferred embodiment, heat storagesubsystem comprises a phase-change heat reservoir device.

The term “thermal transients” as applied to input 110 refers to asubstantial change in heat generation which in traditional heat transfersystems would lead to a concomitant increase in component surfacetemperature. In general, transient heat transfer is heat transfer thatdoes not reach steady state, or which has multiple steady states. Forexample, in the case of large semiconductor microprocessors, it is notunusual for the maximum thermal load to exceed normal operating load byabout 30%. Such thermal transients are experienced by a number ofcomponents in a variety of applications, including, for example,semiconductor devices, optoelectronic devices, thermoelectric coolers,and the like. Those skilled in the art will appreciate, however, thatthe present invention is not so limited, and that the systems andmethods presented herein can be used in any application subject totransient thermal conditions.

Exemplary Embodiments

Having thus given an overview of the present invention, a number ofexemplary embodiments will now be described. It will be appreciated,however, that the illustrated embodiments constitute merely a smallsubset of possible configurations, and that the invention is not solimited.

Referring first to FIG. 2, a heat reservoir in accordance with oneaspect of the present invention includes a heat transfer subsystem 202thermally coupled to heat storage subsystem 203. Without loss ofgenerality, the illustrated embodiment is shown as a heat pipe 202thermally coupled to a phase-change heat storage subsystem 203, whereinsubsystem 203 includes a sealed case 208, a phase-change material 204,and a plurality of fins 206 having heat pipe 202 extending there-throughin what is often termed a “pipe-through-fin” configuration. One end 210of heat pipe 202 is thermally coupled to the heat source generating theheat input 110 shown in FIG. 1.

As is known in the art, a heat pipe is an efficient heat conductortypically consisting of an elongated vessel having a wick structurelining its inner walls and an enclosed working fluid. When one end ofthe heat pipe is heated, the working fluid undergoes a phase change andevaporates from liquid to vapor. The vapor travels through the hollowcore to the other end of the heat pipe, where heat energy is removed bya heat sink or the like. The vapor condenses back to liquid, releasingheat, and the resulting liquid travels back to the first end throughcapillary action via the wick. The working fluid of the heat pipe isselected in accordance with the temperature range of the heat source.Common working fluids include, for example, water, methanol, and liquidammonia. Other working fluids may be selected depending upon theoperational range of the heat pipe.

In this regard, it will be appreciated that these and other generalprinciples of heat-transfer, conduction, convection, and radiation willbe well understood by those skilled in the art, and will therefore notbe described in detail herein. Basic information regarding heat-transfercan be found in a number of references, including, for example,INCROPERA AND DEWITT, FUNDAMENTALS OF HEAT AND MASS TRANSFER, 5th ed.(2001), and HOLMAN, J. P., HEAT TRANSFER, 9th ed. (2002). Additionalinformation regarding the nature of heat pipes may be found, forexample, IN G. P. PETERSON, AN INTRODUCTION TO HEAT PIPES: MODELING,TESTING, AND APPLICATIONS (1994).

It will also be appreciated that, as mentioned above, the presentinvention is not limited to the use of heat pipes. The heat transfersubsystem shown as heat pipe 202 in FIG. 2 may comprise any othersuitable heat transfer component, e.g., a closed-loop liquid path,thermosyphon, a high conductivity carbon fiber material, a highconductivity metal, or the like.

Heat storage subsystem 203 employs a phase change material 204, housedwithin sealed case 208, which changes phases (e.g., from solid toliquid) at a threshold temperature in response to heat input, therebyacting as a heat storage unit or heat capacitor. Phase change material204 is preferably selected based on, among other things, its fusion ortransition temperature. More particularly, in a preferred embodiment,phase change material 204 is selected such that its transitiontemperature is less than the maximum operating temperature of the heatsource being cooled (i.e., the source of heat input 110 in FIG. 1) andgreater than the steady state temperature of the heat source at itsnominal heat emitting state. Depending upon the application, acceptablephase change materials include, for example, various salt hydrates suchas magnesium nitrate, sodium acetate, etc., paraffin, water, methanol,liquid nitrogen, liquid ammonia, and polyalcohols such as pentaglycerineand neopentylglyol.

Sealed case 208 may be configured in any suitable shape and may befabricated using a variety of materials, e.g., plastics such aspolypropelene, EPDM, and polyolefin, and/or metals such as steel,stainless steel, copper, aluminum, and the like.

Fins 206 are thermally coupled to the heat transfer subsystem 202 andare preferably distributed evenly and efficiently within sealed case 208so as to reduce large open areas. The fins may comprise any suitablematerial, for example, copper, aluminum, carbon-fiber, etc. In theillustrated embodiment, fins 206 are thin discs suitably bonded to heatpipe 202, which extends axially through a cylindrical sealed case 208.Fins 206 may be bonded to heat pipe 202 through a variety of knowntechniques, including, for example, direct bond, pressure bond, adhesivebond, epoxy bond, solder bond, brazed bond, interfacing material bond,and the like.

In general, the illustrated system functions as follows. Heat enters thesystem at end 210 of heat pipe 202 (which may include the use of a heatcollector, described in detail below) and traverses heat pipe 202 toheat storage subsystem 203. Under nominal conditions, heat storagesubsystem will absorb a portion of the heat entering the system, causingphase change material 203 to raise in temperature slightly, but notreaching the phase changing temperature of phase change material 204(e.g., the material remains solid).

When the heat load increases and/or the environmental conditions becomemore severe (e.g., through increased ambient temperature, decreasedairflow, etc.) the temperature of all components in the system willincrease. When the temperature of the phase change material 204increases beyond the phase change temperature, the material will beginto melt, and this phase change process consumes extra energy whilemaintaining the temperature at the phase change temperature. Thus, theheat storage subsystem will be maintained at a temperature slightlyhigher than the phase change temperature, while the additional heat willbe stored in the latent heat of the phase change material 204. Any heatdissipation subsystems (not shown in FIG. 2) will continue to dissipatethe heat, but will not be overburdened by the extra heat or more severeenvironmental conditions.

When the system returns to its nominal heat load, the excess heat storedin storage subsystem 203 will be conducted back through heat pipe 202and, eventually, dissipated through any heat rejection subsystemspresent in the system. At this time, the temperature of the systemcomponents will remain higher while there is a mixture of solid andliquid phase change material 204 in heat storage subsystem 203. Once allenergy stored in the latent heat of phase change material 204 has beendissipated, the system returns to its normal operating temperature.

Particular embodiments of the present invention appropriate for anyparticular application may be selected in accordance with three keydesign parameters. First, the amount of heat energy that can be storedby the heat reservoir is dependent upon the volume of phase changematerial 204 encapsulated within heat storage subsystem 203. Second, therate of heat transferred to phase change material 204 is dependent uponthe geometry of heat pipe 202 and the surface area of fins 206. Third,the phase change temperature is dependent upon the material propertiesof phase change material 204. Thus, by varying these three parameters,an efficient heat transfer system can be selected for a particularapplication.

FIG. 3 shows an alternate embodiment further comprising a heat collectorand heat rejection subsystem. Specifically, a heat collector 304 isthermally coupled to heat source 110, heat sink 302, and heat pipe 202.Heat collector 304 (e.g., a plate in direct contact with the heatsource) functions, during nominal conditions, primarily as a method ofdistributing thermal energy to heat sink 302, where it is dissipated inthe conventional manner. At the same time, however, a discrete amount ofheat transfer is effected by heat pipe 202 to heat storage subsystem203. As described above in connection with FIG. 2, after a transientheating event, heat is transferred from heat storage subsystem 203 toheat collector 304 via heat pipe 202, where the extra thermal energy isdissipated (by heat sink 302) and normal operation can proceed. It willbe appreciated that, while the illustrated embodiment depicts a finnedheat sink 302, and the phrase “heat sink” is used herein for thepurposes of simplicity, a variety of other known or futureheat-rejection components may be employed, e.g., radiators, cold-plates,peltier coolers, heat exchangers and the like.

Heat collector 304 may comprise any suitable component capable of beingthermally coupled to the surrounding components. In the illustratedembodiment, for example, heat collector 304 comprises a block ofconductive material (e.g., metal, or a composite material) which can bepositioned in direct contact with the heat source 110. This embodimentwould be particularly applicable in microelectronic applications, wherethe heat source 110 typically consists of a rectangular semiconductorchip having an exposed surface, typically a metal lid, bare die, aplastic, or an epoxy. A variety of thermal adhesives, greases, and/orpads (no shown) may be used to further enhance thermal connectivitybetween heat collector 304 and heat source 110.

FIG. 4 shows another embodiment of the present invention wherein heatsink 302 is positioned between heat collector 304 and heat storagesubsystem 203. Either separate heat pipe segments 202, or a continuousheat pipe 202 are used to provide thermal coupling between heat sink 302and the two antipodal components (i.e., heat collector 304 and heatstorage subsystem 203). Furthermore, additional embodiments may beconceived wherein single or multiple heat pipes may be configured inparallel or serial between one or more of the subsystems.

This configuration offers the further advantage that, for a given heightof heat sink 302, the assembly has an overall lower profile; i.e., thevertical distance between the bottom of heat collector 304 and the topof heat sink 302 is reduced by an amount equal to the thickness of heatcollector 304.

While the configuration shown in FIG. 4 includes two heat pipe segments202 of approximately equal length extending from heat sink 302 about180-degrees apart, the present invention comprehends any other suitablegeometry.

FIG. 5 shows another embodiment for a heat reservoir where, as in FIG.2, the heat pipe 202 is thermally coupled to heat storage subsystem 203,but wherein the heat rejection subsystem 203 is integrated with the heatstorage subsystem 203. More particularly, in the illustrated embodiment,a series of fins 502 are directly attached to the sealed case of heatstorage subsystem 203. This embodiment may or may not be integrated witha second heat sink, depending upon the particular application; that is,the heat reservoir design shown may be substituted into theconfigurations shown in FIGS. 3 and 4, if desired.

FIG. 6 shows another embodiment which includes a centrally-located heatcollector 304 thermally coupled to heat sink 302 via a heat pipe segment202, and thermally coupled to thermal storage subsystem 203 through asecond heat pipe segment 202 or continuous pipe(s) leading from the heatsink. Furthermore, the illustrated embodiment includes a variation in itheat storage system in that a series of axial fins 602 are wrappedaround heat pipe 202. FIG. 8 shows another embodiment, a variation ofthat shown in FIG. 6, wherein axial fins 602 are joined to a thermalbase, which is thermally joined to the heat pipe.

FIG. 7 shows another embodiment, wherein heat storage subsystem 203includes a series of rectilinear fins 702, and wherein heat pipe 202 ismounted to its base. This embodiment is particularly advantageous from amanufacturing point-of-view, as fabrication of the sealed case isindependent of the heat pipe or pipes, allowing the case to become astandard item that is easily integrated into a variety of systems.

It is important to note that a wide variety of geometries are possible.As illustrated, the heat is added to the base, and the heat pipe issimply embedded or laid against the base to form a thermal path. Theheat pipe does not pass through the phase change material in the heatreservoir as it does in other illustrated embodiments.

Furthermore, it should be noted that there are a large variety of finstructures that are possible and valid, such as rectilinear fins, pinfins, radial fins, and circular fins. While fins generally provide aneffective way to conduct the heat throughout the phase change material,in other embodiments the reservoir need not include fins at all.

FIG. 9 shows yet another embodiment, wherein heat storage subsystem 203includes a series of radial pin fins 204 distributed in a suitablepattern around a heat pipe 202, and which may or may not directlycontact heat pipe 202, as described above in connection with FIGS. 6 and8.

SUMMARY

What has been presented is a heat transfer system capable of managingthermal transients. As described above, it is possible to designparticular configurations of the present invention depending upon, amongother things, space limitations and thermal conditions.

Embodiments of the present invention are particularly advantageous inthe following sets of conditions:

-   -   1. Transient heat load with high peak/base load ratio, short        duration peak load, wherein limited space prohibits full peak        load heat sink or radiator plate.    -   2. Transient environment and limited space prohibits full peak        load heat sink, or radiator plate.    -   3. Short duration heat load and no available convection airflow.    -   4. Short duration heat load and no space available for heat        sink.    -   5. Low profile heat reservoir.    -   6. Overheating protection for short duration.    -   7. General Temperature stabilization.

One or more of the above conditions may be present in a variety ofcurrent and future applications, including, for example, TEC driven carseat air conditioning, radiation detector heat loads, missilecomponents, satellite components, lab/medical/scientific instruments,future generations of palm-tops and cell phones, telecom CPU faultprotection, no-fault computers, instruments, controllers, systemsdesigned to prolong shut-down time period or enable sufficient time tostart-up back-up systems, and telecom (and/or electronics) enclosuresrequiring temperature-stabilization under extreme transient high-ambienttemperature conditions.

Other examples where the present invention may be particularlyadvantageous include such items as laptop computers, mobile electronics,personal data assistants and cell phones, thermoelectrically drivencoolers, short-life components, telecom applications, electronic videogame consoles, and the like, where it is necessary to accommodatedramatic transient loads from a heat source such as a semiconductor orintegrated optic device.

As mentioned in the Background section, the traditional heat transfersolution for the above applications would involve selecting a very largeheat sink designed to dissipate the maximum thermal transient generatedby the target component. If there was insufficient space available toaccommodate the design, then additional cooling would need to beprovided through the use of increased air flow or other means.

In contrast, systems in accordance with the present invention wouldallow the engineer to incorporate a much smaller heat sink or use lowerairflow or lower coolant flow, or reside in a higher ambient designed toaccommodate normal heating conditions while at the same timeincorporating a relatively small heat storage subsystem capable ofabsorbing transient heat input.

Although the invention has been described herein in conjunction with theappended drawings, those skilled in the art will appreciate that thescope of the invention is not so limited. For example, while the variousembodiments have been discussed occasionally in the context ofsemiconductor applications and electronic components, it will beappreciated that the invention may be employed in any application wherethe reduction of thermal transients aids the engineer in desiging aheat-transfer solution. Furthermore, it will be apparent that a heatstorage subsystem may be implemented using a wide variety of fin designsand geometries. The figures depict merely a few possible designs. Theseand other modifications in the selection, design, and arrangement of thevarious components and steps discussed herein may be made withoutdeparting from the scope of the invention as set forth in the appendedclaims.

1. A heat reservoir device for managing a heat input from a componentsubject to transient conditions and having space constraints defined bya first volume of available space surrounding said component, said heatreservoir device comprising: a heat transfer subsystem comprising a heatpipe having a first end thermally coupled to said component and a secondend thermally coupled to a heat storage subsystem; a heat rejectionsubsystem coupled to said heat storage subsystem; wherein said heatstorage subsystem is remotely situated from said heat input, outsidesaid first volume of available space, and comprises a phase changematerial capable of changing phases in response to said transientconditions causing the temperature of said phase change material to riseabove its phase change temperature, wherein said heat rejectionsubsystem and said heat storage system have a combined volume that isgreater than said first volume of available space.
 2. The heat reservoirdevice of claim 1, wherein said phase change material comprises amaterial selected from the group consisting of: a hydrated salt, sodiumacetate, magnesium nitrate, paraffin, and water.
 3. The heat reservoirdevice of claim 1, wherein said heat storage subsystem furthercomprises: a sealed case; a plurality of fins thermally coupled to saidheat transfer subsystem and encapsulated by said sealed case, whereinsaid phase change material is thermally coupled to said plurality offins.
 4. The heat reservoir device of claim 3, wherein said plurality offins comprises a series of disc-shaped fins axially distributed alongand connected to said heat transfer subsystem.
 5. The heat reservoirdevice of claim 3, wherein said plurality of fins comprises a series ofradial fins thermally coupled to said heat transfer system.
 6. The heatreservoir device of claim 3, wherein said plurality of fins protrudefrom a base coupled to said heat transfer subsystem.